The iron-binding sites of transferrin can function in a heterogeneic manner, where iron from one of the two sites is surrendered (or accepted) at a cell receptor, or in a homogeneic manner, where exchange occurs randomly (or simultaneously) at both sites. In rat plasma, a further complication arises due to the discovery that normal rat plasma has two isotransferrins existing in a ratio of 1:4. In severe iron-deficiency anemia, nearly equal quantities of each are present. We propose to study both rat isotransferrins in order to determine whether there are sufficient differences in physicochemical and biol gical properties of either isotransferrin to account for in vivo and in vitro observations of heterogeneic behavior or whether this behavior is linked to the individual iron-binding sites. We will investigate the distribution of iron (added as FeNTA, Fe citrate or Fe salts) among the binding sites to ascertain what transferrin components are actually labelled when this form of radioiron is added to plasma. Plasma trnsferrin moieties will be resolved and estimated quantitatively by combined PAGE in 60M urea and immun electrophoresis. PAGE separates apo, diferric and both forms of monoferric. Crossing this technique with Laurel immunoelectrophoresis will then quantitate relative amounts of each moiety.